Op amp increases potentiometer's resolution
Chuck Wojslaw, Xicor Inc, Milpitas, CA, and P Gareth Lloyd, Elab Ltd, Stoke-on-Trent, UK -- EDN, June 5, 2000
A key parameter of digitally controlled potentiometers (DCPs) is the number of taps (n) programmable positions of the wiper. This parameter establishes the resolution in programmable-voltage and -current applications. A number of circuit techniques exist for improving the resolution using one or more DCPs with a given number of taps. The circuit in Figure 1 has no theoretical limit on increasing the resolution in programmable-voltage applications. The IC3 amplifier circuit is an inverting summer with weighted input-resistor values. The input voltages to the summer are the programmable output voltages of the DCPs. To reduce the effects of loading, IC1 and IC2 buffer the signals from the potentiometer wipers. For an n-tap potentiometer, the input resistors of the summing amplifier are R and nR, providing a programmable output voltage of 0V>VOUT>-VREF(1+1/n), with a resolution of VOUT(smallest)=-VREF/[n(n-1)].
For the dual 64-tap Xicor X9418 DCP and the circuit values shown, the output voltage, VOUT, has n2, or 4096, programmable values. The full-scale value is 2.5391V, and the smallest programmed voltage is 0.62 mV. You can program the coarse DCP1 from 0 to 2.5V with a resolution of 39 mV, and you can program the fine DCP2 from 0 to 39 mV with a resolution of 0.62 mV. This circuit provides the same resolution as a 12-bit D/A converter. Measured data fell within 2 LSBs of calculated values. Adding more potentiometers, buffers, and input resistors provides theoretically unlimited resolution. If you add a third section, the resolution increases to one part in 262,144 (18 bits). You can implement a similar scheme using a noninverting summer circuit. You can use the circuit as a substitute for expensive D/A converters in any application that requires a precise, high-resolution programmable voltage. (DI #2537).
